The present document is based on Japanese Priority Document JP 2000-391186, filed in the Japanese Patent Office on Dec. 22, 2000, the entire contents of which being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a resonance type switching power supply unit having a switching circuit for carrying out a switching operation on an inputted direct current, a converter transformer supplied with a switched output from the switching circuit, a resonator including a coil of the converter transformer as a resonator element, a rectifier for rectifying an output of the converter transformer and supplying the resultant output to a load connected to the resonance type switching power supply unit, and a switching controller for controlling the switching frequency of the switching circuit depending on the rectified output deriving from the rectifier.
2. Description of Related Art
A switching power supply unit has been utilized in a situation in which a commercially available alternative current is rectified and smoothed to create a direct current, and this direct current is subjected to a switching operation at a high frequency, such as 100 kHz, and converted into a current having a desired voltage by a transformer at a high efficiency.
A system for controlling the output voltage in the above-described switching power supply unit may be a pulse width modulation control system in which the duty ratio of the switching pulse is controlled depending on the fluctuation of the output voltage. The system for controlling the output voltage in the above-described switching power supply unit may be a frequency control system or a phase control system of a resonance type in which the frequency or the phase of the switching pulse is controlled. Other variations may be possible for the system for controlling the output voltage in the above-described switching power supply unit.
FIG. 8 is a diagram showing a fundamental circuit configuration of a conventional current resonance type switching power supply unit 200.
As shown in FIG. 8, the current resonance type switching power supply unit 200 is arranged to include an AC rectifying unit 3 connected to a commercially available power supply source 1 through a noise filter 2, a smoothing condenser 4 for smoothing the rectified output generated from the AC rectifying unit 3, a switching circuit 5 for carrying out a switching operation on the direct current that has undergone smoothing by the smoothing condenser 4, and so on. A current resonant circuit 6 composed of a primary coil 10A of a converter transformer 10 and a resonant condenser 6C connected in series is connected to the AC rectifying unit 3 through the switching circuit 5. A secondary coil 10B of the converter transformer 10 is connected to a rectifying/smoothing circuit 20, which is composed of diodes 21A and 21B, condensers 22A and 22B, and a choke coil 23. Further, the rectifying/smoothing circuit 20 is connected with a switching control circuit 25 for controlling the switching operation of the switching circuit through an error detecting circuit 24. The rectifying/smoothing circuit 20 also is connected with output terminals 26A and 26B.
According to the above arrangement of the current resonance type switching power supply unit 200, the secondary side voltage is outputted from the rectifying/smoothing circuit 20 at the output terminals 26A and 26B, and the secondary side voltage is compared with a reference voltage Vref by a voltage comparator 24A in the error detecting circuit 24 to create an error voltage. Then, the resultant error voltage is fed back to the switching control circuit 25 through a photocoupler 24B, whereby switching elements 5A and 5B provided in the switching circuit 5 are switched therebetween at a frequency that is varied depending on the error voltage. Thus, even if the input voltage or the load is fluctuated, a stable voltage can always be obtained.
According to the above-described arrangement of the current resonance type switching power supply unit 200, owing to the resonant circuit formed of the leakage inductance le of the converter transformer 10 and the capacity of the resonant condenser 6C, energy loss can be decreased.
According to the conventional current resonance type switching power supply unit 200, the leakage inductance le of the converter transformer 10 and the capacity of the resonant condenser 6C are fixedly determined. The switching circuit 5 creates the minimum switching loss from the switching elements 5A and 5B at a range near a self-resonance frequency fr, and, consequently, the maximum output voltage can be obtained and the loss ratio becomes the minimum due to the operation characteristic. The operation at this time can be illustrated as shown in FIG. 9. That is, when the condition that the input voltage becomes the minimum and the load current becomes the maximum is satisfied, the switching loss becomes the lowest, with the result that the conversion efficiency becomes the highest.
If either the input voltage or the load is increased, then the switching frequency is increased so that the output voltage becomes constant. The operation at this time can be illustrated as shown in FIG. 10. That is, since the switching element 5A is forcibly turned off to cut the current, which is going to flow at the self-resonance point denoted as IQ1, the turning-off operation at a timing when the current value is large results in an increased switching loss.
When the above power supply unit is utilized in a practical situation, however, the input voltage will vary in a range from 100V to 240V, depending on the region where the unit is driven, with the result that the load current will also vary depending on the operation of an apparatus connected to the power supply unit. Further, in an ordinary case, as the input voltage is increased, and also as the load becomes smaller, the switching frequency is increased so that the power converted into one on the secondary side can be saved and the output becomes stable. Accordingly, when the conventional current resonance type switching power supply unit 200 is operated under an ordinary condition, it is not operated at a region where the loss ratio becomes low.
The present invention is made in view of the above problem concerning the above-described conventional current resonance type switching power supply unit. That is, according to the present invention, there is provided a novel current resonance type switching power supply unit, which can always convert an inputted power into one having a desired voltage at a high converting efficiency.
According to the present invention, the leakage inductance of the converter transformer is varied in accordance with the fluctuation of the inputted voltage and the load imposed on the power supply unit, whereby an optimum resonating condition can always be created and the switching loss can be constantly suppressed.
According to the present invention, in order to attain the above purpose, there is provided a resonance type switching power supply unit having a switching circuit for carrying out a switching operation on an inputted direct current, a converter transformer supplied with a switched output from the switching circuit, a resonator including a coil of the converter transformer as a resonator element, a rectifier for rectifying an output of the converter transformer and supplying the resultant output to a load connected to the resonance type switching power supply unit, and a switching controller for controlling the switching frequency of the switching circuit depending on the rectified output deriving from the rectifier, wherein the converter transformer is arranged as one capable of varying the leakage inductance thereof, and the resonance type switching power supply unit includes a detector for detecting an input voltage applied to the switching circuit and an output current supplied from the rectifier to the load and a leakage inductance controller for variably controlling the leakage inductance of the converter transformer.
According to the present invention, it becomes possible to vary the leakage inductance of the converter transformer depending on the fluctuation of the inputted voltage and the load imposed on the power supply unit. Therefore, an optimum resonating condition can always be created and the switching loss can be constantly suppressed.
Accordingly, with the above invention, it becomes possible to provide a resonance type switching power supply unit, which can always convert an inputted power into one having a desired voltage at a high converting efficiency.